Cocaine addiction produces long-lasting alterations in neuroplasticity within the nucleus accumbens core (NAcore) linked to an enduring vulnerability to relapse. Following extinction of cocaine self-administration, cocaine-conditioned cues produce a transient synaptic potentiation (t-SP) of NAcore medium spiny neurons (MSNs), characterized by increased dendritic spine head diameter and AMPA:NMDA ratios. This t-SP also extends to the extracellular matrix, where exposure to cocaine-conditioned cues causes a rapid enhancement of matrix metalloproteinase (MMP) activity, which is required for dendritic spine head enlargement. To date, the mechanism of MMP activation following exposure to cocaine-conditioned cues has yet to be elucidated. MMPs are secreted as an inactive pro-form which must be cleaved or modified prior to activation, and one mechanism for this activation is S-nitrosylation of the MMP pro-domain by nitric oxide (NO). The gaseous transmitter NO is produced by a small population of interneurons that express the enzyme neuronal nitric oxide synthase (nNOS), deemed nitrergic interneurons. Preliminary data presented here demonstrates that cocaine exposure enhanced NAcore nNOS and MMP activity and that inhibition of nNOS activity in the NAcore inhibited reinstated cocaine seeking and the cue-induced induction of MMP activity. I also show that selective activation of Gq signaling in NAcore nitrergic interneurons (with Gq-coupled designer receptors exclusively activated by designer drugs (DREADD)) enhanced MMP activity in-vivo and that nitrergic interneurons receive input from the prelimbic cortex (PL) dorsal raphe (DRN) and the ventral tegmental area (VTA). In the K99 aims, I propose to characterize glutamate and DREADD-evoked NO release in the NAcore following cocaine experience using anesthetized NO electrochemical recordings in rats and transgenic mice, which I have gained experience with during my NIDA T32. I will also selectively modulate the activity NAcore nitrergic interneurons with the DREADD receptor technology in order to determine the impact of activation/deactivation of these neurons on cued cocaine reinstatement. To do this, I will learn mouse self-administration techniques. In the R00 aims, I will perform electrochemical recordings in behaving rats to observe NO dynamics during self-administration, extinction and reinstatement. Training for these experiments will be completed in year 2 of the K99 phase. Finally, I will use NOS1cre knock-in mice and a cre-dependent AAV helper / rabies virus system to express Gq- DREADD in the monosynaptic afferents of NAcore nitrergic interneurons. Following DREADD-mediated activation of neurons in the PL, DRN or VTA I will measure NAcore NO levels in vivo. These experiments have the potential to reveal undiscovered neurobiological mechanisms of cocaine addiction, and could contribute to the development of novel therapeutic options aimed at reversing cocaine-induced neurobiological alterations.